I recently bought some colorful fabric and I was told to pre-wash it before starting a quilting project because the colors were likely to leak. I was also told to wash it with vinegar.

After 3 times, my green fabric is fine, but the purple one keeps losing its dye. Is there anything I can do, or should I just abandon the idea of using that fabric or ever washing the bag if I do it anyway? I'm also not sure about if I'm doing the vinegar thing right. What I've done is I've soaked the fabric in very hot water mised with vinegar, later rinsed it and dried it in the drier.

Hmm, maybe try sending it through a hot wash and a hot drier cycle? Sometimes dye will set better if heated. Your fabric will likely shrink a bit further than it already has. You could also try some of those colour catcher thingies in the wash.

I like to soak darker fabrics in a bowl of 1:1 water and vinegar over night, not just an hour. Then rinse it out until it doesn't smell of vinegar, anymore, and air dry it. I sew clothing for dolls, and the fabric has to be set before it's used, because the dolls I sew for are expensive.

I soak things that I think will run in hot water with vinegar and table salt added (enough to make the room smell like chips) and then wash and dry how ever I plan on washing and drying the item. It works really well.